It is known that many materials, such as carbon/carbon composites, carbon fibers, graphite, and certain metals have properties which make them attractive for use in aerospace and other applications in which their susceptibility to oxidative deterioration at elevated temperatures is a serious disadvantage.
As disclosed in Niebylski's copending U.S. patent applications Ser. No. 261,612, filed Oct. 24, 1988, U.S. Pat. No. 272,258, filed Nov. 17, 1988, U.S. Pat. No. 414,463, filed Sep. 29, 1989, U.S. Pat. No. 415,653, filed Oct. 2, 1989, and U.S. Pat. No. 466,622, filed Jan. 17, 1990, such normally oxidizable materials can be protected from this oxidative deterioration by providing them with ceramic coatings derived from dispersions of certain solids, such as silicon carbide, metal borides, aluminum-silicon eutectic, or silicon metal, in polysilazane solutions.
The utility of the aforementioned coating compositions in forming one or more of the strata of multilayer ceramic coatings on normally oxidizable substrates, such as carbon/carbon composites, is taught in copending U.S. patent applications Ser. No. 446,184 (Niebylski), filed Dec. 5, 1989, and U.S. Pat. No. 466,225 (Niebylski et al.) and U.S. Pat. No. 466,594 (Niebylski), both filed Jan. 17, 1990. In the former two of these applications, it is also taught that, although cracking is apt to occur when ceramic layers derived from polysilazane compositions are applied over ceramic layers derived from organoborosilazane polymer compositions, the cracking tendency can be minimized by the use between those different layers of a buffer layer derived from a composition comprising as the essential components a Group IIIb metal hydrocarbyloxide, a Group IVa metal hydrocarbyloxide, a (dialkylamino)metal of Group IVa, and a polysilazane and/or (dialkylamino)silane.